We describe incidence calculus, a logic for probabilistic reasoning. In incidence
calculus, probabilities are not directly associated with formulae. Rather
sets of possible worlds are directly associated with formulae and probabilities
are calculated from these. This enables incidence calculus to be truth functional,
which a logic based on a purely numeric uncertainty measure cannot
be. This, in turn, enables tighter probablity intervals to be calculated for
theorems of an incidence calculus theory than is possible in a purely numeric
uncertainty theory.